A variety of different signal detectors have been developed in the past with many of these including electronic circuits incorporating tuning circuitry adapted to receive, filter and process signals within a selected frequency range. For signal detectors specifically constructed to monitor a broad range of oscillatory signals, certain problems arise where those circuits seek to monitor signals having certain frequency characteristics. For example, many signal detectors exhibit a high susceptibility to noise and produce unreliable readouts where random noise signals fall within the target frequency range. Also, these systems often encounter problems with signal-to-noise ratio for the target signals versus the background noise. In order to solve these problems, many existing signal detectors resort to superheterodyning in order to increase sensitivity, eliminate noise and shift frequency bands. As noted above, the present invention especially concerns itself with the detection of ultrasonic signals in an ambient sound environment, as well as sonic and ultrasonic signals present in solids. The desirability of detecting ultrasonic signals in an ambient sound environment has increased recently due to the recognition that ultrasonic detectors may readily be implemented as leak detectors to detect ultrasonic signals which, for example, are created by the escape of pressurized gases through small openings. This is useful, for example, in detecting leakage from pipelines as well as detecting airflow paths, for example, through insulation of houses and commercial buildings and through automobile doors and panels. Other analytical values of airborne ultrasonic detectors are being discovered as well, such as bearing wear indicators.
Where there is an internal leak within a system, such as a valve leak, a steam trap, or bearing friction, it is beneficial to employ a touch probe to detect the sound because the sound generated is within a solid. As such, it is necessary to contact the solid with the touch probe in order to detect the sound and then convert it into an electrical signal with an acoustic emission sensor for processing.
In recent years, two types of ultrasonic detectors have prevailed in the market. A first type employs a crystal system to mechanically couple an ultrasonic signal to a local oscillator in order to convert the frequency of the input ultrasonic signal to a resultant signal having a frequency within the audible range. While being relatively inexpensive, crystal based systems have exhibited limited performance and have significant sensitivity problems where the ambient environment is quiet. These crystal based systems are also susceptible to noise and have problems with signal-to-noise ratio. In addition, crystal based systems are susceptible to mechanical vibrations and temperature changes which can affect their sensitivity and yield false readings. Further, crystal based systems often and undesirably respond to infra-sonic and sonic signals that modulate the system so that again faulty readings occur. These crystal based systems also have a very limited frequency range for target signals unless there is an ability to adjust the frequency of the local oscillator within the system.
A second system commonly used employs signal mixers that heterodyne a local oscillator with the input signal. Again, these heterodyne systems are susceptible to noise, have problems with signal-to-noise ratio and exhibit a limited frequency range unless the oscillator frequency is adjustable. While these systems do not exhibit problems due to sonic or mechanical vibrations, they are nonetheless susceptible to temperature changes that can yield faulty readings. Further, systems that employ the heterodyne technique require multi-offset settings and are thus difficult to adjust and maintain over an extended period of use.
A much improved ultrasonic detector has been described in my U.S. Pat. No. 5,103,675 issued Apr. 14, 1992. This ultrasonic detector offers superior performance to those discussed above. In its detailed circuitry, the ultrasonic signal detector described in my '675 patent detects ultrasonic signals by a transducer and converts these signals into an electrical transducer signal that is then filtered to remove undesired frequency components to produce a resulting input signal. Processing circuitry is then responsive to the input signal for producing an intermediate signal that has an intermediate frequency scaled from the input signal frequency into an audible frequency. The intermediate signal is then amplified proportionately to the input signal amplitude to produce a detector output signal that is then displayed.
A still further improved ultrasonic detector is described in my U.S. Pat. No. 5,432,755 issued Jul. 11, 1995, the disclosure of which is incorporated herein by reference. The ultrasonic detector described in my '755 patent is particularly directed to a compact, lightweight and portable unit which is relatively inexpensive to produce. This ultrasonic detector produces a relatively clean audio output of the type typically familiar to users of ultrasonic detectors with a minimum number of components. In its detailed circuitry, the ultrasonic signal detector described in the '755 patent utilizes a visual output of peak received signal strength which is more representative of the received signal, as opposed to an average of such signal strength. The ultrasonic detector also utilizes very sensitive heterodyne circuits employing a telecommunications chip, known as a Gilbert Cell mixer, to blend a very high frequency local signal against a received very high frequency signal, typically in the range of 100 MHz to 140 MHz. The use of Gilbert Cell mixer technology had not previously been recognized as appropriate for ultrasonic detectors due to the relatively low frequencies encountered in the ultrasonic environment. Moreover, it had been thought that the technology embodied in the very high frequency in the telecommunications circuitry could not, in fact, be employed at the lower frequencies of ultrasonic signals. Surprisingly, however, I have discovered that such a circuit design can indeed be employed to produce a very low cost, compact ultrasonic detector which exhibits superior performance.
Despite the advantages of my previous ultrasonic signal detectors, there is a need to provide a new and improved signal detector for monitoring both sonic and ultrasonic signals through the incorporation of two different sensor types associated with the instrument, an integrated acoustic emissions sensor, or touch probe, and an airborne sensor. There is a further need for such a unit to have independent sensitivity and volume adjustment so that the instrument is more discriminating, and thus more versatile, while monitoring leaks. It would also be advantageous for such an instrument to operate in a variety of modes and incorporate an erasable and programmable memory for the retention of user-defined parameters per mode, while at the same time permitting digitally generated and tuned band switching as well as field calibration. The present invention is directed to meeting these needs among others.